It is conventionally known that an engine will be reversely rotated by increased gas pressure in a combustion chamber, and in particular, when the number of revolutions of the engine is relatively low.
As for this problem, in an ignition control device of an engine described in Japanese Unexamined Patent Application Publication No. 2006-207565 to be described below, the reverse revolution of the engine is prevented by forcibly causing a misfire when the number of revolutions of the engine is lower than a threshold value determined according to the present load of the engine. Moreover, according to this device, the threshold value of the number of revolutions of the engine at which the misfire is caused is determined according to the load of the engine, so that forcibly causing a useless misfire can be prevented. A measurement value received from a throttle sensor, for example, is used as the load of the engine.
An output signal from a throttle sensor corresponds to the extent of the throttle opening which in turn relates to the amount of gas pressure present in the combustion chamber and correlates to some extent with the reverse revolution of the engine. However, it is difficult to determine whether a throttle opening will decrease or increase in the future based on the output signal of the throttle sensor. In a state where the throttle opening will decrease in the future and in a state where the throttle opening will increase in the future, even if the number of revolutions of the engine is the same, the probability that the engine will be reversely revolved may be different. However, the ignition control device disclosed in Japanese Unexamined Patent Application Publication No. 2006-207565 does not consider the difference in probability between the two states and hence does not necessarily predict the reverse revolution of the engine correctly.